This invention relates to relative humidity sensitive materials and, in particular, to materials comprising a mixture of certain salts and certain metals or metal oxides in an appropriate binder.
There is a long established need to both measure and control the relative humidity in both home and industrial environments. Whether the need is for an indication of the ambient relative humidity or for control of the relative humidity, it is first necessary to provide a transducing instrument or sensor having characteristics which depend upon the ambient relative humidity. In the past, many hygrometric instruments have depended upon dimensional variations in various materials to indicate the relative humidity. Historically significant among such devices is the well-known hair hygrometer which depends for its operation on the expansion and contraction of a fiber such as hair. These dimensional changes are then employed to move a mechanical linkage assembly which is typically connected to a mechanical dial indicator. Other hygrometric devices depend for their operation upon swelling of the material upon exposure to high level relative humidity. Again dimensional changes associated with this swelling are typically converted to appropriate mechanical movement for indicating the degree of swelling and consequently the level of relative humidity associated therewith. Many of these devices are unreliable and inaccurate. Additionally, hygrometric materials based upon dimensional changes frequently undergo long term degradation in performance following repeated cycling from low to high relative humidity and back again.
More recent relative humidity sensors are based on changes in electrical characteristics of materials in response to exposure to environments exhibiting a range of relative humidity conditions. These hygrometric materials typically exploit the variation in electrical resistance exhibited by these materials. For example, in U.S. Pat. No. 3,703,697, issued Nov. 21, 1972 to Nicholas, there is apparently described the relative humidity sensor in which an iron oxide powder (Fe.sub.2 O.sub.3) is fired onto a substrate and treated with a solution of ferric chloride (FeCl.sub.3). The Nicholas' relative humidity sensor appears to be based solely upon the variation of the resistance of this treated material and it further appears that it is the object of Nicholas to treat the iron oxide with ferric chloride to reduce the range of variation over which the resistance varies in accordance with variations in the ambient relative humidity. However, it is also seen that the reduction in the range over which the resistance varies through the formation of oxychlorides results in distortion of the otherwise linear graph of resistance (on logarithmic scale) versus relative humidity (on a linear scale).
For certain applications in which humidity sensors are desired in feedback control systems, the object of which is to maintain the relative humidity at a fixed or in a narrow range of temperatures, it is highly desirable to be able to produce a relative humidity sensor which is particularly responsive in the desired region of control. Conventional relative humidity sensors and materials, including those of Nicholas above, do not appear to be amenable to treatment which seek to adjust their responses to fit specific relative humidity ranges.
Other relative humidity sensors also appear to be based on the electrical properties of certain metal oxides. In particular, U.S. Pat. No. 3,345,596, issued Oct. 3, 1967 to Delaney et al. appears to disclose the use of cobalt oxide as such a material. This sensor appears to be produced by firing a substrate containing cobaltic oxide (Co.sub.3 O.sub.4) deposited on a substrate to produce cobaltous oxide (CoO) having a highly crystalline surface. The resultant sensor appears to produce a variation in resistance in response to relative humidity changes over a four to five order of magnitude range. As indicated in the above-mentioned Nicholas patent, such a dynamic range of variation is undesirable in terms of the electronic circuitry which must conventionally interface with such a device to provide an indication of the relative humidity. Additionally, there is no disclosure in the patent of Delaney et al. to indicate how a hygrometric sensor may be tailored to various humidity ranges.